Narrow UVB-Emitted YBO₃ Phosphor Activated by Bi³⁺ and Gd³⁺ Co-Doping

Y_0.9(Gd_xBi_1-x)_0.1BO₃ phosphors (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0, YGB) were obtained via high-temperature solid-state synthesis. Differentiated phases and micro-morphologies were determined by adjusting the synthesis temperature and the activator content of Gd³⁺ ions, verifying the hexagonal phase with an average size of ~200 nm. Strong photon emissions were revealed under both ultraviolet and visible radiation, and the effectiveness of energy transfer from Bi³⁺ to Gd³⁺ ions was confirmed to improve the narrow-band ultraviolet-B (UVB) (⁶P_J→⁸S_7/2) emission of Gd³⁺ ions. The optimal emission was obtained from Y_0.9Gd_0.08Bi_0.02BO₃ phosphor annealed at 800 °C, for which maximum quantum yields (QYs) can reach 24.75% and 1.33% under 273 nm and 532 nm excitations, respectively. The optimal QY from the Gd³⁺-Bi³⁺ co-doped YGB phosphor is 75 times the single Gd³⁺-doped one, illustrating that these UVB luminescent phosphors based on co-doped YBO₃ orthoborates possess bright UVB emissions and good excitability under the excitation of different wavelengths. Efficient photon conversion and intense UVB emissions indicate that the multifunctional Gd³⁺-Bi³⁺ co-doped YBO₃ orthoborate is a potential candidate for skin treatment.